1. Field of the Invention
The present invention relates to an automatic focusing apparatus for a camera, which performs focus adjustment in accordance with a so-called hill-climbing scheme.
2. Description of the Related Art
A so-called "hill-climbing" type automatic focusing scheme is known as a conventional scheme wherein an image picked up by a photographing optical system is formed on an image pickup element, a predetermined frequency component is extracted from the image signal output from the image pickup element, and the photographing optical system is moved to a position where an output amplitude of the extracted frequency component becomes maximum, thereby performing focus adjustment.
An automatic focusing apparatus for a television camera which employs the above hill-climbing scheme is described in NHK Technical Report, Vol 17, No. 1 (Serial Number 86), pp. 21-37, 1965.
An automatic focusing apparatus employing the hill-climbing scheme described in the above reference will be described with reference to FIGS. 38 and 39.
FIG. 38 is a block diagram showing an arrangement of the above automatic focusing apparatus. An object image picked up by a photographing optical system 1 is focused on an image pickup element 2 and is photoelectrically converted into a video signal. This video signal is output from the image pickup element 2 and is amplified by a preamplifier 3. The amplified signal is input to a band-pass filter (to be referred to as a BPF hereinafter) 4, and the BPF 4 extracts a predetermined frequency component. An output amplitude of the extracted frequency component is detected by a detector 5 and a peak detector 6. Output amplitude characteristics of the video signal frequency component extracted by the BPF 4 are shown in FIG. 39. As shown in FIG. 39, the output amplitude of the video signal has a maximum value at an in-focus position. The output amplitude having this characteristic curve is held by a sample/hold circuit 7 in units of fields of the video signal. The sampled/held signal is output to a one-field delay circuit 8 and a comparator 9. The comparator 9 compares a value previously held and supplied from the one-field delay circuit 8 with a value currently held and supplied from the sample/hold circuit 7. The comparator 9 controls a motor drive circuit 10 to move the photographing optical system in a direction to increase the output amplitude on the basis of the comparison result. The photographing optical system 1 is moved to the in-focus position by a motor 11.
For example, in the focusing apparatus, when the amplitude value of the previously held output has a level A (FIG. 39) and the amplitude value of the currently held output has a level B (FIG. 39), the comparator 9 determines condition A&lt;B. In this case, the comparator 9 controls the motor drive circuit 10 to continuously move the photographing optical system 1 in the same direction as described above, so that the photographing optical system comes close to the in-focus position. When the output amplitude value supplied from the sample/hold circuit 7 to the comparator 9 reaches the E level, the comparator 9 determines condition D&gt;E. That is, the comparator 9 determines that the photographing optical system 1 has passed the in-focus position. The drive direction of the motor 11 is reversed, and the photographing optical system 1 is moved in the direction of the in-focus position. These operations are repeated to reciprocate the photographing optical system 1 near the in-focus position. When a steady state is obtained, focusing is completed.
Another conventional focusing method is a phase correlation scheme for performing focusing adjustment by using two light beams which pass different pupil positions of a photographing optical system. This phase correlation scheme is described in, e.g., Minolta Techno Report (1986).
FIG. 40 is a block diagram showing an arrangement of an automatic focusing apparatus employing the phase correlation scheme. Referring to FIG. 40, reference numeral 21 denotes a photographing lens. An object image picked up by this photographing lens 21 is temporarily focused on a focusing plane F and is then focused on image pickup elements 23a and 23b by separator lenses 22a and 22b again. Reference numerals 24a and 24b denote aperture masks having functions of causing light beams having specific f-numbers to pass through. Image signals obtained by photoelectric conversion by the image pickup elements 23a and 23b are amplified by preamplifiers 25a and 25b, respectively. The amplified signals are A/D-converted by A/D converters 26a and 26b, respectively. The obtained digital signals are then input to a microprocessor 27. The microprocessor 27 calculates an interimage distance d of the object images respectively formed on the image pickup elements 23a and 23b. The microprocessor 27 outputs a control signal to a motor drive circuit 28 to drive a motor 29, thereby performing focus adjustment. In this case, when a focusing state is a near-focus state, the interimage distance is short. To the contrary, when a focusing state is a far-focus state, this distance is long. The microprocessor 27 calculates an interimage distance by using a correlation algorithm and detects a defocus amount and a focusing direction.
In the automatic focusing apparatus employing the hill-climbing scheme shown in FIG. 38, output amplitude values of the video signals phase-shifted by one field are compared, and a one-field period is required for one focusing cycle. The focusing period of the automatic focusing apparatus for a camera is excessively long.
The output amplitude of the frequency component extracted by the BPF 4 does not always provide a smooth curve by causes such as electrical noise, hand vibrations, and an abrupt change in object (e.g., an object instantaneously passes across the field). As shown in FIG. 41, for example, the amplitude curve has a plurality of peaks P1 to P4. When focus adjustment is performed by the hill-climbing scheme using such an output amplitude curve described above, the peaks P1 to P4 are undesirably determined as in-focus positions, thus greatly degrading focusing precision.
In the automatic focusing apparatus employing the phase correlation scheme, incident light to be incident on predetermined areas is cut by the aperture masks 24a and 24b, and an amount of incident light is greatly reduced. For this reason, a charge accumulation time of the image pickup elements 23a and 23b must be prolonged to obtain an optimal exposure value for a dark object. At the same time, the photographing optical system cannot be driven during charge accumulation, and the focusing operation requires a longer period.
In the phase correlation operation, an accurate interimage distance cannot be obtained for an object having a cyclic pattern, and focusing precision is undesirably degraded.
Published Unexamined Japanese Patent Application No. 61-32669 discloses a method of adding horizontal scanning line signals preceding and succeeding an image having a low S/N ratio (e.g., a dark image) to increase the S/N ratio of the signal. This method, however, may have a disadvantage of a decrease in S/N ratio of the signal when the preceding and succeeding horizontal scanning lines have a low degree of correlation.
In order to cope with a case wherein the amplitude curve of the output from the BPF 4 has the plurality of peaks P1 to P4, a means is disclosed to detect a change in object by a change in aperture value, interrupt focus adjustment for a predetermined period of time when the aperture value is abruptly changed, and then start focus adjustment again, as disclosed in Published Unexamined Japanese Patent Application No. 58-215176. Unexamined Japanese Patent Application No. 61-107312 discloses a means for monitoring a change in evaluation function sampled every predetermined period, and detecting a change in distance between a lens and the object when the evaluation function is changed with a predetermined level by n (n.gtoreq.2) times. The means disclosed in these prior art specifications require a long focusing period.